1. Field of the Invention
The present invention relates to a distance measuring apparatus used in photographing equipment such as cameras and, more particularly, to a new and improved distance measuring apparatus capable of measuring a distance to an object present within a measurable range at high speed in a short period of time even if the object is not located at the center of the photographing field of view.
2. Related Background Art
In recent years, cameras incorporate automatic focusing systems. An active infrared distance measuring apparatus employing the principle of trigonometric measurement is known as a major component of the automatic focusing system.
A conventional infrared distance measuring apparatus comprises a light-emitting element such as an infrared light-emitting diode and a light-receiving element for receiving an infrared beam emitted from the diode and reflected by an object and for outputting an electrical signal. The conventional infrared distance measuring apparatus can perform distance measurement with relatively high precision and is compact. From these advantages, the infrared light measuring apparatus is used as a distance measuring apparatus suitable for a camera.
In a conventional distance measuring apparatus incorporated in a camera, a beam from the light-emitting element is designed to be projected onto only the central area of the photographing field of view. For example, if a user uses the camera incorporating the conventional distance measuring apparatus to take a picture of two persons standing having the center of the photographing field of view between them, the beam emitted from the light-emitting element is projected on a distant position (i.e., infinite position whose distance from the camera is not measurable) of the background. As a result, the two persons as objects to be photographed are not kept in the just-in-focus state and are thus out of focus since no object is present at the center of the field of view.
In order to eliminate the above disadvantage, distance measurement is performed after the central position in the photographing field of view is aligned with an object to be photographed. The user then determines framing and takes a picture of this object. However, this photographing technique is very difficult for a user who does not satisfactorily understand the automatic focus control mechanism. In addition, the photographing technique is time-consuming and cumbersome.
In order to solve the above problem, a distance measuring apparatus employing a wide field of view is proposed wherein the photographing field of view is divided into a plurality of areas and distance measurement data values from these areas are evaluated to determine focusing of the camera.
There are two types of distance measuring apparatuses employing the wide field of view. These apparatuses have the following disadvantages. According to the first type of apparatus disclosed in Japanese Patent Disclosure (Kokai) No. 143914/1984, a plurality of light-emitting elements for respectively emitting beams onto a plurality of areas within the photographing field of view are time-divisionally driven, and output signals from light-receiving elements are time-divisionally processed to calculate distance measurement values of the respective light emission points. A proper distance measurement value is output after the measurement values are processed according to a predetermined algorithm. According to the first type of apparatus, an accurate distance measurement value can be obtained. However, the circuit arrangement of the apparatus is bulky and high cost and requires a long processing time, as compared with other conventional distance measurement apparatuses. Therefore, a long period of time is required from the beginning of distance measurement to the end of photographing.
According to the second type of apparatus as described in Japanese Patent Application No. 152297/1985, the following distance measurement operation is performed. In this distance measuring apparatus, distance measurement is performed for one of the central and peripheral areas within the photographing field of view. If the measured value of the measured area is an infinite valve, precise distance measurement is performed for the other one of the central and peripheral areas. If the measured value is a finite value, this value is output as the distance measurement value.
The second type of distance measurement apparatus is smaller than the first type but requires two distance measurements. In this sense, the distance measurement time is doubled as compared with the conventional distance measuring apparatus and cannot be conveniently used in practice. The disadvantage of this apparatus is decisive in a distance measuring apparatus employing a double integrator as a signal processor. In the distance measuring apparatus of this type, a long signal processing time is required to perform double integration.